Helicopters using horizontal rotors have been known for a long time. They allow an aircraft to move vertically (allowing vertical take-offs), hover in the air, move side to side, etc. The use of horizontal rotors gives helicopters an unprecedented amount of movement in relation to a fixed wing craft.
However, conventional helicopters are typically very complex. Most conventional helicopters use a large horizontal rotor for lift and a smaller vertical rotor (the tail rotor) to counterbalance torque imposed on the helicopter by the rotation of the large lift rotor. By altering the pitch of the blades of the small vertical rotor, the entire helicopter can be pivoted from side to side or held straight.
The horizontal rotor must also be specially designed to cause the helicopter to tilt in different directions when required and to control the amount of lift created by the rotors. In one common conventional system, a swash plate assembly, comprising a fixed swash plate and a rotating swash plate, is used to change the pitch angle of the rotor blades. The swash plate assembly can be used in two ways: to change the pitch angle of all of the rotor blades collectively; or, by changing the pitch angle of the rotor blades individually and cyclically as they revolve. By changing the pitch angle of all of the rotor blades collectively, the amount of lift generated by the helicopter can be increased or decreased causing the helicopter to ascend or descend, respectively. By changing the pitch angle of the rotor blades cyclically as they revolve, the lift created on one side of the rotor can be increased causing the helicopter to tilt in a desired direction and thereby move in the direction the helicopter is tilting.
Tandem coaxial rotors have been developed to avoid the use of a smaller vertically mounted rotor. A pair of horizontal rotors rotating in opposite directions around a single axis are used. The counter-rotating pair of horizontal rotor blades can be used to balance out the torque created around the single axis by each of the two rotors and by altering the speeds of the two rotors relative to each other, the helicopter can be yawed left or right around the axis shared by the rotors.
While these tandem coaxial rotors remove the necessity for a tail rotor (vertical rotor) to counterbalance the rotational forces placed on a helicopter by a single rotor, to achieve all the desired movements of a conventional helicopter helicopters with tandem coaxial rotors have increased the mechanical complexity of the rotor systems. Rather than in more conventional systems which use two swashplates in the swashplate assembly to change the pitch of the rotor blades, tandem coaxial rotors typically use two swashplates for each rotor requiring four swashplates to be needed. In addition, provisions typically have to be made for the control system of the upper rotor to pass through the lower rotor control system.
While some remote controlled helicopters such as toys and drones have used simple versions of tandem coaxial rotor systems, they have often sacrificed the range of producible movements in order to reduce the mechanical complexity of the rotor system.
It is desirable in many applications to have a helicopter that can achieve all the movements of a conventional helicopter with a reduced mechanical complexity. It is also desirable to have a helicopter that can be folded into a relatively compact form for transport.